1. Field of the Invention
The present invention relates to a transistor that uses an oxide semiconductor layer, a semiconductor device including the transistor, and a manufacturing method of the transistor and the semiconductor device.
2. Description of the Related Art
There are various kinds of metal oxides, which are used for a wide range of applications. Indium oxide is a well-known material and used as a material of a transparent electrode which is needed in a liquid crystal display or the like.
Some metal oxides exhibit semiconductor characteristics. In general, metal oxides serve as insulators; however, it is known that metal oxides serve as semiconductors depending on the combination of elements included in the metal oxides.
For example, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like can be given as metal oxides exhibiting semiconductor characteristics, and thin film transistors in which a channel formation region is formed using such a metal oxide exhibiting semiconductor characteristics are already known (Patent Documents 1 to 4, Non-Patent Document 1).
As the metal oxides, not only single-component oxides but also multi-component oxides are known. For example, InGaO3(ZnO)m (m: natural number) having a homologous series is known as a multi-component oxide semiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4).
Further, it is proved that the oxide semiconductor formed using an In—Ga—Zn based oxide as described above can be used for a channel layer of a thin film transistor (also referred to as a TFT) (Patent Document 5, Non-Patent Documents 5 and 6).
However, semiconductor characteristics are likely to vary because of damage to the oxide semiconductor due to an etchant or plasma or entry of an element such as hydrogen to the oxide semiconductor in an element manufacturing process. Accordingly, problems of variation and deterioration in electrical characteristics of the element are caused.